Introduction to Fungi, Third Edition

366 HYMENOASCOMYCETES: PYRENOMYCETES
hyphae form a lining layer around this cavity
and develop centripetally towards the centre of
the cavity. Croziers develop at the tips of the
ascogenous hyphae and produce a succession
of asci, indicating the ascohymenial nature of
the fungus. The asci have extremely thin walls.
There are no paraphyses and no periphyses.
Ascospores released into the cavity of the
perithecium are extruded through the narrow
neck and accumulate in a mucilaginous blob
held in place by the ring of ostiolar hairs. Barkboring
beetles which feed on the ascospores
(and also conidia) help in their dispersal. The
beetles are attracted by ‘fruity’ odours emitted
from the mycelium. These are volatile metabolites,
mainly short-chain alcohols and esters, as
well as monoterpenes and sesquiterpenes
(Hanssen, 1993).
Some species of Ophiostoma have two or
more synanamorphs. These vary in structure
from yeast-like to mononematous or synnematous.
They have been assigned to several
anamorph genera including Leptographium,
Sporothrix, Pesotum and Graphium. For example,
O. ulmi has a yeast-like anamorph, a mononematous
anamorph referred to Pesotum and a synnematous
anamorph, Graphium ulmi. Older hyphae
may also produce endoconidia (Fig. 12.37).
Wingfield et al. (1991) have placed Pesotum in
synonymy with Graphium.
12.6.2 Dutch elm disease
Dutch elm disease is a vascular wilt disease
of Ulmus spp. caused by O. ulmi, O. novo-ulmi and
O. himal-ulmi. It is best regarded as a disease
complex because it is invariably associated with
the activities of bark-boring scolytid beetles such
as Scolytus scolytus, S. multistriatus and Hylurgopinus
rufipes. These are the vectors for the disease.
Elm populations worldwide have been ravaged,
causing the death of millions of trees in Europe
and North America and changing the appearance
of the landscape, particularly where hedgerow
elms have been killed. Diseased tree leaves
wilt in dry weather and rapidly turn brown
and brittle. Defoliation and death of the twigs
ensues, and eventually the whole tree dies
(Plate 5h). The fungus persists as a saprotroph
in the bark of dead trees. Infected twigs show
a characteristic brown flecking in the sapwood.
This is associated with the development of
brown-coloured bladder-like inflated cells of the
xylem parenchyma called tyloses, which invade
the xylem vessels and block them. Gums released
partly by the action of cell wall-degrading enzymes
of the pathogen impede water flow. Wilting
is also associated with the production of the
wilt toxin cerato-ulmin, which is a hydrophobin
(Richards, 1993). However, since mutants of
O. novo-ulmi with low cerato-ulmin production
still retain pathogenicity, an alternative role
for it has been sought. Temple et al. (1997) have
shown that cerato-ulmin can enhance the adhesiveness
of yeast-like propagules of the pathogen
and also protect them from desiccation.
Dutch elm disease was first described in
Holland in the late 1920s and spread to the rest
of north-western Europe, North America and to
parts of Asia. The disease declined in severity
in Europe in the 1940s but persisted in North
America, possibly because the American elms
were more susceptible. This first pandemic was
relatively mild in effect and did not destroy
the elm tree populations. When first discovered,
the disease was associated with the synnematal
conidial state, Graphium ulmi. Later the
teleomorphic state was discovered and named
Ceratostomella ulmi, then Ceratocystis ulmi, now
O. ulmi. In the mid-1960s simultaneous outbreaks
of a more severe and aggressive form of the
disease occurred, centred around ports in
southern England and originating from
infected elm logs from North America. These
outbreaks spread rapidly from the original
infection foci into much of Britain and mainland
Europe.
Isolations from trees affected by the aggressive
form of the disease yielded a strain of
Ophiostoma distinguished by its fluffy appearance
and more rapid growth in culture, in contrast
with the waxy appearance and slower growth
of the non-aggressive strain. In many places
where O. ulmi was present, it has now been
replaced by the more aggressive form (Brasier
et al., 1998). The aggressive strain is regarded as
a distinct species, O. novo-ulmi (Brasier, 1991b).
Closer investigation of isolates of O. novo-ulmi,